The trend of oil products consumptions requires an extended conversion of heavy fractions to light products. Many techniques have already been proposed for this purpose, but their application is made difficult by the high contents of Conradson carbon, asphaltenes and metals of said charges.
Thus, the conventional catalytic refining, cracking and hydrocracking processes are not directly applicable, in view of the quick poisoning of the catalysts. Deasphalting of residues provides effectively an oil of low asphaltenes and organometallic compounds content which is convenient for the above-mentioned catalytic treatments but a sufficient economic return by this production process requires the upgrading of asphalt, particularly by conversion to lighter products. It is precisely the object of the invention to provide an improved process for converting such residues.
The processes using a mere thermal treatment such as thermal cracking or coking are not either convenient since they give a low yield of distillable hydrocarbons which, moreover, are of bad quality, and a high yield of coke or pitch, difficult to upgrade. Different solutions have been proposed to improve the quality of the formed products and to reduce the formation of pitch or coke. A first way consists of a thermal cracking in liquid phase in the presence of a hydrogen donor diluent, at a temperature of 370.degree.-538.degree. C. with a residence time of 0.25-5 hours (U.S. Pat. Nos. 2,953,513 and 4,115,246).
A second way consists of a quick heating of the heavy residue, at a temperature of 600.degree.-900.degree. C., under a hydrogen pressure higher than 5 bars, for a time shorter than 10 seconds, followed with a quenching, so as to avoid recombination of the cracking products (U.S. Pat. Nos. 2,875,150 and 3,855,070). In spite of the improvements obtained by these innovations, substantial amounts of coke or pitch are formed, for which an upgrading method remains difficult to find.
It has already been proposed to gasify these final residues, coke or pitch, by reaction with steam and oxygen to produce the hydrogen required for the preceding treatments. The applicant, in particular, has described in U.S. Pat. No. 4,405,442, a process for converting heavy oils to light products integrating said different steps. Although it offers many advantages with respect to the prior art processes (complete conversion of the heavy oil with a high yield of liquid hydrocarbons), this process has the disadvantage of using oxygen in the coke oxy-steam-gasification step, which is conducted at high temperature (900.degree.-1500.degree. C.). This oxygen addition supplies heat to the gasification zone, by partial combustion of the coke, and for the endothermicity of the steam-gasification reaction. However, addition of oxygen results in technological complexities and hence requires heavy investments, particularly for the oxygen producing unit.
On the other hand, it is known that alkali, alkaline-earth and transition metals, mainly as carbonates, hydroxides and oxides, catalyze the gasification of carbon and/or carbonaceous materials by steam and/or carbon dioxide (see for example the paper of Taylor and Neville J.A. C.S. 1921, 43, pages 2055 and following).
By using these catalysts it is possible to substantially reduce the temperature at which gasification takes place, for example to 600.degree.-800.degree. C. instead of 900.degree.-1500.degree. C. in non catalytic processes.
The thermodynamic balance at such lower temperatures also contributes to make the gasification process less endothermic, so that the heat required for gasification may be supplied by other means than oxygen injection.
One of these means consists, for example, of circulating a solid heat carrier between the coke gasification zone and the hydropyrolysis zone, for transferring a part of the heat generated by the hydropyrolysis exothermic reactions, to the gasification zone.